1. Field of the Invention
The present invention relates to a placement optimization system aided by CAD (computer aided design) determining layout in a printed circuit board or a semiconductor substrate.
2. Description of the Related Art
A layout design system for printed circuit board or semiconductor substrate is realized typically by determining placement of elements to be mounted so as to make the area after layout small as far as possible and interconnecting those placement elements.
In determining positions of placement elements to be handled, there has been conventionally used a method of defining the estimated total wiring length and distribution of interconnecting wiring as items to be evaluated and of improving placement so as to minimize an evaluation function value calculated by using an evaluation function formed by combining the above described items. Items such as the estimated total wiring length among placement elements and wiring distribution are thus defined as component items of the placement evaluation function on the basis of the assumption that minimizing those values results in a minimized area of the final layout result.
Although there is correlation between the estimated total wiring length and the final area, for example, however, the correlation is not necessarily satisfied at all times. Even if the evaluation function value is minimum, the area is not minimum in some results. Further, distribution of interconnection wiring is detected by evaluating the area indirectly by means of a simple accumulator, the result of minimum area cannot be obtained accurately. Further, since a plurality of items which do not directly represent object properties are combined to perform evaluation, valid coefficients for determining evaluation weights of respective items change with the scale and characteristics of the circuit to be handled. In order to obtain a set of weight coefficients which become optimum with respect to the subject circuit, it is necessary to determine the set of coefficients by means of laborious experiments or experience.